Approximately 100 cases have been described in the literature to date.

The facial features are characteristic and include

- Deep set eyes

- Strabismus

- Myopia

- Marked nasal root

- Broad and/or beaked nasal bridge

- Prominent Cupid's bow

- Everted lower lip

- Tented upper lip

- Large mouth

- Widely spaced teeth

- Wide and shallow palate

- Ears with thick and overfolded helix

Most have a smiling appearance.

Intellectual disability is severe. Language is absent or limited to only a few words. Stereotypic movements particularly of the arms, wrists and fingers is almost universal. Hypotonia is common (75%) as is an unsteady gait. All have delayed walking. Other features include a single (simian) palmar crease, long, slender fingers, flat feet and cryptorchidism (in males). Finger clubbing and the presence of fetal pads is common. Hyperventilation occurs in over half and is frequently followed by apnea and cyanosis. During these episodes aerophagia may occur. Constipation is common. Microcephaly and seizures may occur. Hypopigmented skin macules have occasionally been reported.

LFS is clinically distinguished from other X-linked forms of intellectual disability by the accompanying presence of marfanoid habitus. Marfanoid habitus describes a group of physical features common to Marfan syndrome. Including Marfan syndrome and LFS, marfanoid features of this type have also been observed with several other disorders, one of which is multiple endocrine neoplasia type 2.

In LFS, specific features identified as marfanoid include: a long, narrow face; tall, thin stature; long, slender limbs, fingers and toes (not unlike arachnodactyly) with joint hyperextensibility, shortened halluces (the big toes) and long second toes.

The diagnosis of marfanoid habitus in LFS is often delayed because many of the physical features and characteristics associated with it are usually not evident until adolescence.

Most of the signs of MWS are present during the neonatal period. The most common signs at this state are multiple congenital joint contractures, dysmorphic features with mask-like face, blepharophimosis, ptosis, micrognathia, cleft or high arched palate, low-set ears, arachnodactyly, chest deformation as pectus, kyphoscoliosis and absent deep tendon reflexes are frequent minor malformations have also been described and consist of renal anomalies, cardiovascular abnormalities, hypospadias, omphalomesenteric duct, hypertriphic pyloric stenosis, duodenal bands, hyoplastic right lower lobe of the lung, displacement of the larynx to the right and vertebral abnormalities, cerebral malformations.

- 75% of children with MWS have blepharophimosis, small mouth, micrognathia, kyphosis/scoliosis, radio ulnar synostose and multiple contractures.

- They have severe developmental delay; congenital joint contractures and blepharophimosis should be present in every patient

- 2 out of 3 of the following signs should be manifested: post natal growth, mask-like faces, retardation, and decreased muscular mass.

- Some may require additional signs such as; micrognathia, high arched or cleft palate, low set ears, kyphoscoliosis.

- The symptoms of MWS are normally diagnosed during the newborn period

Psychopathology and related behavioral abnormalities are typically seen in LFS, and they may be considered in the diagnosis of the disorder. The most common of these in LFS is an autism-like spectrum disorder, and LFS is considered as one of a number of genetic disorders associated with autism. Additional alterations of psychopathology with behavioral manifestations that have been observed in LFS include: psychotic behavior, schizophrenia, hyperactivity and attention-deficit hyperactivity disorder, aggression, oppositional defiant disorder, obsessive compulsive disorder, extreme shyness, learning disability, cognitive impairment, short-term memory deficit, low frustration tolerance, social dysfunction, lack of impulse control, eating disorder and associated malnutrition, attributed to psychogenic loss of appetite; and pyromania.

While psychiatric conditions like these are to be expected with LFS, there have also been cases of the disorder with some preservation of mental and behavioral abilities, such as problem solving, reasoning and normal intelligence.

The psychopathology of LFS usually exhibits schizophrenia. When schizophrenia is diagnosed in an individual known to be affected by intellectual disability, LFS may be considered in the differential diagnosis of schizophrenia, with confirmation of cause through appropriate psychiatric and genetic evaluation methods.

Recognised symptoms up till now are:

- Autism or autistic behaviors

- ADHD

- Learning disability

- Large head

- Dysmorphic facial appearance - mild

- Prominent forehead

- Wide-set eyes (hypertelorism)

- Schizophrenia

- Loose joints

- GERD

- Sleep disturbances

- Sleep Apnea

- Underdeveloped parts of brain - corpus callosum and cerebellar vermis

- Neuroblastoma

- Speech & developmental delays

- Chiari malformation of the brain

- Congenital heart defects

- Hypotonia

It is not clear whether the list of symptoms is complete. Very little information is known about the syndrome. The symptomology may be different among individuals, even in the same family.

Neu-Laxova syndrome presents with severe malformations leading to prenatal or neonatal death. Typically, NLS involves characteristic facial features, decreased fetal movements and skin abnormalities.

Fetuses or newborns with Neu–Laxova syndrome have typical facial characteristics which include proptosis (bulging eyes) with eyelid malformations, nose malformations, round and gaping mouth, micrognathia (small jaw) and low set or malformed ears. Additional facial malformations may be present, such as cleft lip or cleft palate. Limb malformations are common and involve the fingers (syndactyly), hands or feet. Additionally, edema and flexion deformities are often present. Other features of NLS are severe intrauterine growth restriction, skin abnormalities (ichthyosis and hyperkeratosis) and decreased movement.

Malformations in the central nervous system are frequent and may include microcephaly, lissencephaly or microgyria, hypoplasia of the cerebellum and agenesis of the corpus callosum. Other malformations may also be present, such as neural tube defects.

This syndrome consists a number of typical features. These include

- Agenesis of the corpus callosum (80-99% patients)

- Hypopigmentation of the eyes and hair (80-99% patients)

- Cardiomyopathy (80-99% patients)

- Combined immunodeficiency (80-99% patients)

- Muscular hypotonia (80-99% patients)

- Abnormality of retinal pigmentation (80-99% patients)

- Recurrent chest infections (80-99% patients)

- Abnormal EEG (80-99% patients)

- Intellectual disability (80-99% patients)

- Cataracts (75%)

- Seizures (65%)

- Renal abnormalities (15%)

Infections of the gastrointestinal and urinary tracts are common. Swallowing and feeding difficulties early on may result in a failure to thrive. Optic nerve hypoplasia, nystagmus and photophobia may occur. Facial dysmorphism (cleft lip/palate and micrognathia) and syndactyly may be present. Sensorineural hearing loss may also be present.

Death in infancy is not uncommon and is usually due to cardiac complications or severe infections.

Microlissencephaly Type B or Barth microlissencephaly syndrome: is a microlissencephaly with thick cortex, severe cerebellar and brainstem hypoplasia. The Barth-type of MLIS is the most severe of all the known lissencephaly syndromes.

This phenotype consists of polyhydramnios (probably due to poor fetal swallowing), severe congenital microcephaly, weak respiratory effort, and survival for only a few hours or days. Barth described two siblings with this type as having a very low brainweight, wide ventricles, a very thin neopallium, absent corpus callosum and absent olfactory nerve.

Microlissencephaly with mildly to moderately thick (6–8 mm) cortex, callosal agenesis

The natural history of MWS is not well known: many patients died in infancy and clinical follow-up has been reported in few surviving adults. However, diagnosis may be more difficult to establish in adults patients, such as: blepharophimosis, contractures, growth retardation, and developmental delay, whereas minor face anomalies are less noticeable as the patient grows older. Throughout the development of the patient from young child to older adult changes the behavior drastically, from kindness to restless and hyperactive to aggressive.

FG syndrome's major clinical features include intellectual disability, usually severe; hyperactive behavior, often with an outgoing personality; severe constipation, with or without structural anomalies in the anus such as imperforate anus; macrocephaly; severe hypotonia; a characteristic facial appearance due to hypotonia, giving a droopy, "open-mouthed" expression, a thin upper lip, a full or pouting lower lip, and partial or complete loss of the corpus callosum. About a third of reported cases of individuals with FG syndrome die in infancy, usually due to respiratory infection; premature death is rare after infancy.

The incidence of this condition is <1 per million population. It is found only in females as all affected males die before birth. Teeth with large roots (radiculomegaly), heart defects, and small eyes (microphthalmia) are the characteristic triad found in this syndrome.

Typical features of the condition include:

- Face

- Deep set eyes

- Broad nasal tip divided by a cleft

- Eyes

- Microphthalmia (small eyes)

- Early cataracts

- Glaucoma

- Teeth

- Radiculomegaly (teeth with very large roots)

- Delayed loss of primary teeth

- Missing (oligodontia) or abnormally small teeth

- Misaligned teeth

- Defective tooth enamel.

- Heart defects

- Atrial and/or ventricular defects

- Mitral valve prolapse

- Mild mental retardation and conductive or sensorineural hearing loss may occur.

Proximal 18q- causes developmental disabilities. Receptive language skills tend to be more advanced and can be near normal; however, expressive language skills are usually significantly delayed.

Symptoms begin in infancy and include:

- hypotonia

- areflexia

- amyotrophy

- variable degrees of dysgenesis of the corpus callosum

- mild to severe intellectual and developmental delay

- psychiatric problems including paranoid delusions, depression, hallucinations and autistic-like behavior

Dental features:

- small teeth in males

- pointed (screwdriver shaped or conical) incisors (sometimes called Hutchinson teeth)

- incisors with an irregulal incisal edge

- canines: enlarged and globular; may be dome or bud shaped with trilobed edge

- premolars and molars: small, round and globular; may have supernumary lobes (mulberry or lotus flower shape)

- widely separated teeth (diastemma)

- hypoplastic enamel

- dental agenesis

- presence of mesiodents (median incisor behind normal upper incisors)

- pulp chamber anomalies

Facial features:

- anteverted pinnae

- long face

- prominent nasal bridge and nose

- prognathism occasionally

Ophthalmic features:

- bilateral congenital nuclear opacities (100%)

- severe amblyopia

- nystagmus (93%)

- strabismus (43%)

- microcornea (96%)

- congenital glaucoma

- scleral staphylomas

- retinal cystoid degeneration

- microphthalmia

These lead to severe visual impairment in affected males.

Other:

- The fourth metacarpal may be shortened

30% of patients also have some degree of intellectual impairment: of these 80% are mildly to moderately affected: the other 20% may have developmental delays and behavior problems.

Carrier females display milder variable symptoms of disease. Ocular signs are present in 90% of heterozygous females. These are typically lens opacities often involving the posterior Y sutures. More rarely dental anomalies and the characteristic facial features may also occur.

Genitopatellar syndrome is a rare condition characterized by genital abnormalities, missing or underdeveloped kneecaps (patellae), intellectual disability, and abnormalities affecting other parts of the body.

Genitopatellar syndrome is also associated with delayed development and intellectual disability, which are often severe. Affected individuals may have an unusually small head (microcephaly) and structural brain abnormalities, including underdeveloped or absent tissue connecting the left and right halves of the brain (agenesis of the corpus callosum).

Associated with agenesis (loss) of the corpus callosum, intellectual disabilities are common among individuals with FG syndrome. Motor ability is also impaired as a result of having FG syndrome and its effects on the development of neurons. During infancy, problems arise in the gastrointestinal and gastroesophageal systems of the body. The most common gastrointestinal problems include constipation from imperforated anuses and gastroesophageal reflux. Cardiopulmonary defects also contribute to roughly 60% of premature deaths in infants with FG syndrome. Of all of the congenital heart defects septal defects are the most common. After infancy, long term survival has been recorded to individuals surviving beyond the age of 50.

Children with proximal 18q- are often small for their age. Unlike distal 18q-, however, growth hormone deficiency has not been reported in anyone with proximal 18q-.

Hypotonia is a common finding. Around 10% of people with distal 18q- have seizures.

Children are most commonly identified with Aicardi syndrome before the age of five months. A significant number of girls are products of normal births and seem to be developing normally until around the age of three months, when they begin to have infantile spasms. The onset of infantile spasms at this age is due to closure of the final neural synapses in the brain, a stage of normal brain development. A number of tumors have been reported in association with Aicardi syndrome: choroid plexus papilloma (the most common), medulloblastoma, gastric hyperplastic polyps, rectal polyps, soft palate benign teratoma, hepatoblastoma, parapharyngeal embryonal cell cancer, limb angiosarcoma and scalp lipoma.

Common signs of Say–Meyer syndrome are trigonocephaly as well as head and neck symptoms. The head and neck symptoms come in the form of craniosynostosis affecting the metopic suture (the dense connective tissue structure that divides the two halves of the skull in children which usually fuse together by the age of six). Symptoms of Say–Meyer syndrome other than developmental delay and short stature include

- Intellectual disability.

- Low-set ears/posteriorly rotated ears

- Intellectual deficit as well as learning disability

- Intrauterine growth retardation (poor growth of a baby while it is in the mother's womb)

- Posterior fontanel

- Premature synostosis of the lambdoid suture (the fusion of the bones to the joint is premature)

- Narrow forehead

- Trigonocephaly (a frontal bone anomaly that is characterized by a premature fusion of the bones which gives the forehead a triangular shape)

- Hypotelorism or hypertelorism (reduced or increased width between the eyes)

- Craniosynostosis (when one or more seam-like junctions between two bones fuses by turning into bone. This changes the growth pattern of the skull)

- Low birth weight and height

The affected patients sometimes show a highly arched palate, clinodactyly (a defect in which toes or fingers are positioned abnormally) and ventricular septal defect (a heart defect that allows blood to pass directly from left to the right ventricle which is caused by an opening in the septum). Overall, Say–Meyer syndrome impairs growth, motor function, and mental state.

Heart abnormalities are present in 25–35% of people with distal 18q-. The majority of these defects are septal. Congenital orthopedic anomalies are also relatively common, particularly rocker-bottom feet or clubfoot. Cleft lip and palate are relatively common in people with distal 18q-. Kidney abnormalities have also been reported and include horseshoe kidney, hydronephrosis, polycystic kidney, and absent kidney. Boys with distal 18q- may have genital anomalies, the most frequent being cryptorchidism and hypospadias.

Symptoms vary, but usually result in dysmorphisms in the skull, nervous system, and developmental delay. Dysmorphisms in the heart, kidneys, and musculoskeletal system may also occur. An infant with complete trisomy 9 surviving 20 days after birth showed clinical features including a small face, wide fontanelle, prominent occiput, micrognathia, low set ears, upslanting palpebral fissures, high-arched palate, short sternum, overlapping fingers, limited hip abduction, rocker bottom feet, heart murmurs and also a webbed neck.

Trisomy 9p is one of the most frequent autosomal anomalies compatible with long survival rate. A study of five cases showed an association with Coffin–Siris syndrome, as well as a wide gap between the first and second toes in all five, while three had brain malformations including dilated ventricles with hypogenesis of the corpus callosum and Dandy-Walker malformation.

Oculocerebrocutaneous syndrome (also known as Delleman–Oorthuys syndrome) is a condition characterized by orbital cysts, microphthalmia, porencephaly, agenesis of the corpus callosum, and facial skin tags.

Acrocallosal syndrome (also known as ACLS) is a rare autosomal recessive syndrome characterized by corpus callosum agenesis, polydactyly, multiple dysmorphic features, motor and mental retardation, and other symptoms. The syndrome was first described by Albert Schinzel in 1979.

It is associated with "GLI3".